14.0.0 REST API Documentation updates

[renatonascalves]

• [x] After the code from #490 is released to BuddyPress, update the https://developer.buddypress.org/bp-rest-api/reference/signup/ page with the new endpoint.
• [x] After the code from #489 is released, update this page and this page.
• [x] https://github.com/buddypress/BP-REST/pull/503

[renatonascalves]

• https://developer.buddypress.org/bp-rest-api/reference/signup/#resend-signup-activation-email
• https://developer.buddypress.org/bp-rest-api/reference/signup/
• https://developer.buddypress.org/bp-rest-api/reference/private-messaging/#retrieve-a-specific-messages-thread